Work in progress concerns daily injection of ralaxin to ovariectomized or hypophysectomized-ovariectomized rats for 20 days. Other hormones in conjunction with relaxin include estradiol, progesterone, prolactin, and somatotropin. Mammary gland measurements are wet weight, dry fat-free weight, DNA, RNA, and whole-mount lobule-alveolar development. Relaxin at 20 or 90 GPU enhances some mammary growth in ovariectomized but not hypophysectomized rats. Relaxin synergizes with estradiol and/or progesterone to enhance mammary growth in ovariectomized rats. It also suppresses mammary RNA to some extent, suggesting that it acts with progesterone to suppress lactation prior to parurition. Earlier work has shown prolactin and somatotropin to be effective mammotrophs in hypophysectomized rats. Combinations of these with relaxin, estradiol, and progesterone are now under investigation.